Angle modulated wave detector



Dec. 2l, 1948. I Y w. l.. CARLSON l 2,456,915

'ANGLE MODULATED WAVE DETECTOR I Filed Mayv 22, 1945 2 Sheets-Sheet l INVENTOR %Nafz Ansa/v.

+1 Bvgm ATTORNEY o A im D@n 21, 194s. w. L CARLSON 2,456,915

` ANGLE IDDULATED HAYE DETECTDB Filed llay 22, 1945 2 Sheets-Sheet 2 fmf i AWQRNEY Patented Dec. 21, 1948 2,456,915 ANGLE MoDULATED WAVE DETECTOR Wendell Le Carlson, Princeton,

N. J., assigner to yRadio..Corporation of America, a corporation .Application May 22, 1945, Sera'lLNo. '595,209

.l 14 Claims.

.1, wlpresentuinvention .relates to rnovel detectors for; angleamodiilated high 'frequency waves, and morefianticularlyltoamovelmethod of, and .means fomwdemodulating frequency modulated `(FM) carrier-'moves qIn thetpriorfmethods-:of demodulating FM carricr wav-es, or more: 'generally angl-e modulated waves;itl'xeredias.y usually been employedopposed remmers-.having:applied .thereto signal voltages oflfacommon instantaneous frequency but vof diiferinfg magnitude andipolarity. 'Ifhe generic term "amaremodulatedn comprehends jfrequency modulation, phase modulation or hybrid modulation pcss'ciistng.` cl'ia-racteristicsv common. to both of these., yWhere Ja simple-rectifier. has been emplvyedifto detect FM waves, the .rectifier has been prcviedfmthfafslopinglter network so that the symmetrical FM wave could be detected lalong thexmostiflineariportion ofthe filter slope.

tttiszandmportantfobjectxof my present invention'itogprovide anfimprovedmethod of and vapparatusf-forfdetecting anglemodulatedwaves, where-y inrffiihere.fislrderivedifrom ther-latter a wave whose fonnxor; shapelflasla variable degree of asymmetry dependent on ithe angle modulation, the derived wave :being xrectied :by Ya symmetrical, Ynon-linear detectorfto providers. 2voltage #representative yof the afonesaidrvariable asymmetry.

.dn'fcarxfyingiout Imy invention, Ifempl'oy an detcctorcsystem which` is characterized by the inclusion .ofen .harmonicfgenerator :to produce an hanmunic -ofithe signal wave, iev-phase shifter network y for :providing phaseshifts between'V 1 the fundamentalandfharmonic waves, Vand a symmctricalynonelinear detector which .recti-es the resultant.. -c'f' the fundamental and'- harmonic waves.

.Aidunthertcbjectloffmy invention .is to provide an IEM detector whichais .unresponsive 'to voltages of .less :than iafthr'eshold value :but :fior larger voltagesffresponds vin'ieithrer onefof 'two opposite Ypolarities 'frdependent (on. the -sense .of f variation of the receivedfreqrncy from @the mean frequency. To thisv fend,- I-:ffurtherf 1employ oppositely connected rectmers, :biased 4substantially equally against operation byxvoltages .less..than'the threshold value.

.Another ohjectiofrmy invention Ais to provide a high frequeneyrsigfnal detector `which is'characterizedltby its "inputfvol-tage vs. 'output current. char'nctcnisticibeing symmetrical land Ahaving substantial maturation sections` fat f, the -l opposite ends thereof ltiti-l1-.otherifeatures lwill best .-be. understood by reterencetaithe .following description, 4taken in cotmerctionv withY thee-drawing, .in which .1 have indicated diagrammatically several circuit organizations whereby my invention may be carried .into effect.

In thedrawings:

Fig. 1 is a circuit diagramv of an FM detector embodying one form of. the invention;

Figs. 2a, 3a and 4a.. respectively show phase relations between fundamental .andsecond harmonic wave forms .for different. frequency devia- .tionsof the'signal;

Figs. 2b, 3b .and 4b show the respective .resultant wave forms derived from Figs. 2a, 3a and 4c;

Fig. `5 shows an "input voltage vs. voutput current. characteristic for the detector of Fig.. 1;

Fig..6 -shows-amodication of the detector circuitv of Fig. 1;

Fig. 7 .shows a further modification of the invention; and` Fig. `8 depicts the detection characteristic for ythe fcircuit of Fig. .7.

Referring now to Fig. 1, there is shown the circuit connections of only so much-of an FM receiver .as is necessary to a proper understanding of the present invention. It is assumed that 'the receiver is of the fsuperheterodyne type.

Those skilled in the artv of radio .communication are fullyavvare ofthe various networks required prior to ithe intermediate frequency (LF.) amplier L In the present FM broadcast band of42 to 5D.megacycles (mc.) the cannel width-assigned to eac-h FMstation is .200 kilccycles (kc.) and .the permissible frequency deviation of the transmitted wave is 'l5 kc. ,My Apresent `invention may be employed ,for reception ofy vsuch a transmitted wave, but it is tc be understood that the invention .-is. in no Way Vrestricted to the present FM broadcast band, nor to zsuch particular channel` width. Assuming -for .the .purposefofspecicillustration, that the,receiving1system.-shown1in Fig. 1 lis adjusted ato' receive a station in the broadcast band of vi2-'50 mc., the-collected FM signal Wave may'rst be selectively amplified at carrierifrequency and then converted to `intermediate frequency. The .mean intermediate frequency Imay be,=for eXample,-5.mc. It will ibeundersto'od that the .input terminalsxof the I. F. amplifier lare connected either directly o-rthrough Ipreceding I F. amplifiers, vto `the output terminals of a suit-` able y.prior converter network.

The-ampliedfI; F.energy is applied to the input electrodes `of .the tube 2 -through a tuned I. F. transformer. 3. -It will be understood that the primaryand secondary :resonantfcircuits of /transformer-3 are-each tuned.v tothe :operating I. F.; and that the `pass `band width. of transformer 3`is spectively are operated at a relatively low positive voltage, say +75 volts, so as to provide plate circuit limiting. If desired, the grid current flowing through the resistor Ii' in the grid circuit of the limiter tube may be employed to provide automatic volume control (AVC) voltage for the signal grids of prior signal amplifier tubes. The filter network 'I is inserted in the AVC line 8 to eliminate all pulsatingv current components. Those skilled in the art of radio communication are fully aware of the specific control connections between lead 8 and the controlled amplifiers. The FM detectionsystem of my invention is interposed between the limiter plate circuit and a suitable modulation signal amplifier not shown. It is to be understood that the succeeding modulation signal amplifier may include one or more stages of amplification terminated by a suitable reproducer. Before describing the functioning of the circuitelements in the detection or demodulation system, the specific circuit connections thereof will be described in detail.

While I have referred to the tube 2 as preferably functioning to provide amplitude limiting of the FM signal waves, I also employ such tube as an amplifier and a harmonic generator. The plate circuit of the tube includes a pair of parallel resonant circuits 9 and I, and these circuits are connected in series relation.v Primary circuit 9 is magnetically coupled to a secondary `3circuits, reference is first made to 1Figs.=2a'and resonant circuit II,l whereas primary resonant circuit I'Il is magnetically coupled to the secondv ary resonant circuit l2. On the assumptionl that the mean intermediate frequency is5 mc., each of circuits 9 and VII is tuned to 10 mc.,` the second harmonic of the operating I. F. Each of resonant circuits I0 and I?v is tuned to the fundamental frequency, i. e. 5 mc. Also, the coupling between circuits 9 and II and the pass band width of such circuits are chosen to pass efficiently a frequency band vof I 150 kc., and the magnetic coupling between circuits Ill and I2 is chosen so as to pass eciently a frequency band of 175 kc. In other words, the pass band width of network 9, Il is double'that of the network I0, I2. It is known that frequency variations of signal wave.

theory of operationv the grounded negative and positive terminals respectively of voltage sources I1 and I8. The

voltage sources I1 and I8 may be otherwise arranged so that the anode of each of the diodes I3, I and I5, I6 is negatively biased relative to its cathode.

The condenser 2| derives from across yload resistor I9 the audio frequency voltage developed thereacross due to rectification of the' resultant The condenser 2| may feed one or more audio frequency amplifiers followedk by a reproducer. In Fig. 5 there is shown an illustra tive input voltage vs.v output curren characteristic of the FM detector circuit. It will be noted that there exists a considerable horizontal section A over which zero output current is produced by the detector despite amplitude variations in the voltage impressed on it. Such variations may be negative and positive input voltage variations corresponding to small variations of the intermediatefrequency above andbelow' its mean frequency. This horizontal -Ysection A` is due to the bias voltage applied toy eachrectiiier: f1

`The input voltage exceeding the-biasffon'lone-f i rectifier, due to variation of the intermediate frefi quency in one direction, causes `the rise infout'- put current along section B, while the `inputir' voltage on the opposite cycle-of 'the'.variation of the intermediate frequency causes increase-inf? output current along section C when the voltage" exceeds theA bias on detector characteristic is, therefore, symmetriell|` cally non-linear. f :n I do not wish to be restricted to a particular'l of my invention, particularly-Vi' as to details of such operation. I now believe the following to be a correct explanationofits manner of operation. c

In analyzing the action of the phase shifting?" 2b. The curves D and E represent idealize'dwave forms of the respective'fsignal"voltagesl across secondary circuits I2 and Il when the signals applied to the primary circuit .Ill are'fof* the second harmonic frequency will be twice those of the fundamental frequency.

The circuits 9, II and I0, I2 provide a phase shifting network producing a resultanty wave of variable asymmetry. The detector comprises a pair of oppositely poled rectiers. Anode I3 and cathode I4 provide one diode rectifier, while n the frequency to which circuits Il), I2 are tuned, 'f1 e! g. 5 mc. That is, curve Disthe wave Iform'of the 5'mc. fundamental, and curve E 'is thefwave'* form of the 10 mc. second harmonic. Y relation takes into account the usual Vlshift in phase between primary and secondary of coup-'if led resonant circuits tuned to the applied-free* quencies. A 90shift of phase atthe funda-l"-v mental and at the second harmonic' causes a` net relative phase shift of 45. The amplitudefof-f the second harmonic voltage is preferablyQ-'and'v normally will be, less than the amplitude ofthe "l voltage of fundamental frequency, as is also indi-f cated in Fig. 2a. The resultant of wavesD and E is depicted by curve F in Fig.-2b." The curve F is symmetrical, and' issecured by adding the wave forms D and E in the 'usual-"- manner. It is evident from Fig. 2a that-at 'tne' resonant condition (i. e. when' the fundamental;v and second harmonic frequencies are 5' mcfand* l0 mc. in the' example given) the resultant' sig-1* nal wave applied to theopposed 'rectifiers' I'3', lI'4 and I5, I6 4is symmetrical.A Since'the detection characteristic is symmetrical about ythe zeroV iiiput voltage line, the net output across load resisf: tor I9 must be zero. This follows fron'ithe fact"" that on positive cycles of wave le"v the anode I3"`v becomes positive when the bias due tovoltag-"e' source I1 is overcome. Accordingly; "current ows through condenser 20 in one directioifn On" the negative half cycles of wave F'the cathode" 'the second rectifier.'r Theil i 7. spaced electron deflection electrodes" 32 and 33 are connected to opposite sides cuits l and ll. to the high potential side of circuit l l, while lead 35 connects electrode 33 to the lower end of tuned circuit l0. Electrodes 32 and 33 are at apositive potential relative to the grounded electronv emitter 3|. The normal path of beam 36 is shown by the horizontal dash line. Normally, the electron beam 36 passes midway between electrodes 32 and 33, and grazes the top edge of output electrode 31. The latter has applied to it the '-l-B voltage from the direct current power supply source (not shown). The anode or output electrode 38 is connected to the +B terminal through the output resistor 39 shunted to ground by I. F. bypass condenser All. The condenser 4I feeds the audio frequency signals to the utilization network.

Fig. 8 shows as an "input voltage vs. output current detection characteristic of Fig. 7. It will be seen that the upper and characteristic are flat. The detector tube, therefore, saturates at the positive and negative half cycles of the input voltage at the input network. The resultant wave (curve F of Fig. 2b) between electrodes 32 and 33 will not disturb the normal path of beam 35 lwhen the waves D and E are in in-phase relation (Fig. 2a). However, this produces a certain output current at anode 38, shown by point M at curve G, G' in Fig. 8. In other illustration the beam tubel lower sections G and Gr of thev of resonant cir- Lead'34 connects electrode 32 words, the distribution of electrons between output electrode 38 and 31 for wave F is to produce output current M. If,

now, the wave F (Fig. 3b) exists at the input circuit, then electrode 32 has a tendency to pull beam 3B higher up along anode 38 permitting more electrons to reach the anode. Of course, if the beam 36 is pulled high enough the output current begins to flatten out (section G) due to electrons passing over the top edge of anode 38.

Should the resultant wave applied between deflection electrodes 32 and 33 assume the wave form F" (Fig. 4b), then the electron beam 36 would be pulled from itsnormal path toward electrode 33. This means that-the anode, current would begin to decrease by virtue of the screening effect of shield electrode 31. again, the anode current would decrease until.

Here,

shield or screen electrode the saturation condition G were reached. It I will, therefore, be seen that in the modification of Fig. '7 there exists inherent amplitude limiting action by virtue of the fact that the phase shifter network is not responsive to changes in amplitude of the signal waves. There, also, eX- ists limiting action at the beam tube so far as maximum frequency deviations of the signal wave is concerned. In other words, the beam degree inherently limits the audio frequency output to the maximum frequency swing of the signals. It should be understood that the beam tube; may operate with the input double transformer circuit shown in Figs. 1 and 6, if preferred.

While I have indicated and described a system for carrying my invention into effect, it will be vapparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but thatv -f many modications may be made without de -v parting from the scope of my invention.'v What I claim is:

1. In a method of detecting angle.r modulatedhigh frequency waves, deriving from-the Waves", a fundamental and an harmonic angle modus 'responsive to said ant wave output 8 lated' wave,- combining said fundamental and harmonicwaves to produce a resultant high frequency wave which has' a wave form having a variable degree of asymmetry dependent on the angle modulation, and subjecting the resultant wave to symmetrical, non-linear detection to provide a voltage representative of the said variable asymmetry.

2. In a receiver 'of angle modulated carrierlfundamental and second harmonic wavesdeveloped respectivelyin saidrst and second resonant circuits, said phase shifts depending on the angle modulation, and a detector having a symmetrical, non-linear detection characteristic for rectifying the resultant of Ithe fundamental and second harmonic waves.

Y3. In combination,`in a receiver, a source of frequencyfmodulation signals.`a harmonic generator connected to said source and responsive to the signals thereof to provide the lfundamental and second harmonic components of the frequency modulation signals,

network connected to the generthe relative phases between the fundamental and second harmonic components in accordance withv the frequency deviations of the signals, anda symmetrical non-linear detector responsive tothe output of the phase shifter for producing a rectied voltage corresponding to the aforesaid phase shifts.

4. In combination with a source of frequency modulated carrier waves, a harmonic generator a phase shifter ator for varying responsive to said waves to produce a harmonic 'i the carrier waves, a phase shifter network generator for 'providing phase shifts between the fundamental and harmonic of the carrier waves which depend on the frequency metrical non-linear detector lresponsive to said phase shifter for detecting the resultant of the fundamental and harmonic of said waves.`

5. In a system for detecting angle modulated carrier waves, means for deriving from the angle modulated waves a modulated carrier wave whose f wave form has a variable degreeof'asymmetry dependent on the angle modulation but independentof the amplitude ofthe angle modulated wave, rectifier means coupled to the deriving means and having a symmetrical detection characteristic, and said rectifier means rectifying the derived wave to provide a signal voltage representative of the angle modulation.

6. In combination with 'a source of frequency modulated carrier waves, al tube for generating an harmonic of the waves including input and output electrodes,

cuit coupled to said output electrodes and northe second harmonic frequencyof the waves, a

pair ofropposed diodes including respective {de-'1v bias means, connections applying the resultof 'the' two :phase shifternetffj lay generator, said outfrequency modulation i deviation of the carrier Waves, and a sym- 'e said input electrodes being coupled to the source, a firstphase shifter cir- Works to said opposed diodes, and means for deriving from the opposed diodes a rectified voltage representative of the frequency deviations of the signal waves.

7. A frequency modulation detector system comprising an harmonic generator, a symmetrical non-linear demodulator circuit, and means coupled between the output of said generator and the input of said demodulator circuit for irnpressing the fundamental and second harmonic of frequency modulated carrier waves to be detected on said circuit from which modulation frequencies are derived, said means causing the envelope of the combined fundamental and harmonic frequencies to be symmetrical for positive and negative cycles at the center frequency and to cause the envelope to become non-symmetrical for frequencies deviating from center.

8. In a frequency modulation detector system,

an harmonic generator adapted to have frequency modulation signals applied to the input thereof, a symmetrical non-linear detector circuit from which modulation frequencies are derived, and means coupled between the output of said generator and the input of said detector circuit for applying to said detector circuit fundamental and second harmonic waves derived from said signals, said applying means being constructed and arranged to cause the envelope of the resultant of said fundamental and harmonic lwaves to be symmetrical for positive and negative cycles at the center frequency of the signals and to cause the envelope to become non-symmetrical for frequencies deviating from the center frequency.

9. In a system for detecting frequency modulated high frequency waves, means for deriving from the waves a fundamental and an harmonic frequency modulated wave having a resultant high frequency wave which has a wave form having a variable degree of asymmetry dependent on the frequency modulation, and symmetrical non-linear detection means coupled to said deriving means to provide from the resultant wave a voltage representative of the said variable asymmetry.

10. In a receiver of frequency modulated carrier waves, a limiter and harmonic generator provided with an input circuit tuned to the carrier frequency, means for applying frequency modulated carrier waves to said input circuit, an output network connected to said harmonic generator, said output network including a resonant circuit tuned to said carrier frequency and a second resonant circuit in series therewith tuned to the second harmonic of said carrier frequency said output network being constructed for providing phase shifts between the fundamental and second harmonic waves developed respectively in said rst and second resonant circuits, and a pair of opposed rectiers having a symmetrical nonlinear detection characteristic for rectifying the resultant of the fundamental and second harmonic waves.

11. In combination, in a frequency modulation receiver, means to provide the fundamental and second harmonic components of the frequency modulation signals, a phase shifter network coupled to and responsive to said means for varying the relative phases between the fundamental and second harmonic components in accordance with the frequency deviations of the signals, and a pair of like-biased opposed diodes coupled to said shifter network providing a symmetrical non-linear detection characteristic for producing a rectified voltage corresponding to the aforo-- said phase shifts.

12. In combination with a source of frequency modulated carrier waves, a harmonic generator constructed to produce in response to the source waves a second harmonic of the carrier waves, a phase shifter network coupled io the generator output and constructed to provide phase shifts between the fundamental wave and harmonic wave which depend on the frequency deviation of the carrier waves, and an electron beam tube coupled to the output of said shifter network for detecting the resultant wave of the fundamental and harmonic waves.

13. In a system for detecting angle modulated carrier waves, means for deriving from the angle modulated waves a fundamental and an harmonic angle modulated wave having a resultn ant modulated carrier wave ywhose wave form has a variable degree of asymmetry dependent on the angle modulation but independent of the amplitude of the angle modulated wave, a pair of opposed rectiiiers coupled to the deriving means and having a symmetrical non-linear detection characteristic for rectifying the derived resultant wave to provide a signal voltage representa tive of the angle modulations, means for biasing said rectiers, and means responsive to the intensity of said waves for varying in like manner said biasing means.

14. In combination with a source of frequency modulated carrier waves, a tube for generating an harmonic of the waves including input and output electrodes, said input electrodes being coupled to the source, a first phase shifter circuit coupled to said output electrodes and normally tuned to the fundamental frequency of the waves, a second phase shifter network in circuit with the first phase shifter and normally tuned to the second harmonic frequency of the waves, a pair of opposed diodes including bias means responsive to wave intensity, connections apply ing the resultant wave output of the two phase shifter networks to said opposed diodes, and means for deriving from the opposed diodes a rectified voltage representative of the frequency deviations of the signal waves.

WENDELL L. CARLSON.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name D-ate 2,302,834 Bliss Nov. 24, 1942 2,356,201 Beers Aug. 22, 1944 2,383,855 Hansel] Aug. 28, 1945 

